1. Fields of the Invention
The present invention relates to a fluorescent lamp end-of-life protection circuit in an illuminating electric appliance.
2. Related Technology
A fluorescent lamp is a Green light source, which is energy-saving and high-efficient, and the color-temperature of which are capable of being controlled, and it has been widely used in various fields and become a preferred choice of man-made light sources in daily life. It""s lamp tube, however, may reach its end of life as the ageing of the fluorescent lamp.
There exist the following four cases in respect of end-of-life characteristics of the lamp tube and respective hazard; (1) The lamp tube leaks gas but the filament electrodes at both ends of the lamp tube are undamaged. In this case, the lamp tube cannot be started, but a preheating current is still supplied to the filaments by a ballast, and this may lead to a sharp increase of the temperature of a cathode. When the temperature is too high, the glass wall at the ends of the filament electrode may be melt or the connector at the holder of the lamp may be damaged, thus serious results such as fire may occur and at the same time, the ballast may out of work as excessive output power. (2) A cathode at one end of the lamp tube cannot emit electrons when the lamp tube is in use. In this case, because the temperature in one end of the lamp filament increases rapidly and extra power consumption is consumed, the ballast may out of work. Since the extra power consumption is centralized in the small area in the front of the cathode and is extremely unbalance, this may lead to overheating of the lamp holder, the glass wall, and the connector of the lamp. In the case, an asymmetric voltage appears across the lamp tube, which is referred to as xe2x80x9crectifying effectxe2x80x9d. (3) When the lamp tube is in use, both cathodes thereof cannot emit electrons, and the temperature of the lamp filaments at both ends of the lamp tube increases rapidly. In addition to the increase of the extra power consumption to menace the electronic ballast, the lamp tube voltage is also very high so as to result in the same result as that of the second case described above. (4) The lamp tube is lit, but the tube voltage rises rapidly, for example, due to dirt particles leaked into the lamp tube. If the ballast can sustain discharging, i.e., the ballast can supply normal current to the lamp tube, the power of the lamp tube will increase symmetrically as the rise of its voltage. Though not being localized, the extra power may still lead to overheating of both the lamp tube and the parts of the ballast.
In sum, the hazard mentioned in the case (1) can be avoided by a protection circuit of the ballast, and the cases (2), (3), and (4) can be classified into two types: rectifying effect and overvoltage of the lamp tube.
Only a certain case of the end-of-life characteristics of the lamp tube can be protected in the prior art, such as overvoltage protection of the lamp tube. As the first method, oscillating is stopped when the voltage arrives a set value, and this method is suitable for such a case in which a ballast is used for a lamp tube. As the second method, when the tube voltage arrives the set value, the tube which reaches its end of life is cut off, and this method is suitable for such a case in which a ballast is used for a plurality of lamp tubes. A signal for rectifying effect is acquired after rectifying the lamp tube voltage in bridge type. A thyristor is turned on when rectifying effect occurs in the lamp tube, therefore, the lamp tube in which rectifying effect occurs is shorten through a capacitor, and the lamp tube voltage is reduced in order to turn off the lamp tube. Since the signal is acquired by rectifying, this design works only for the symmetric overvoltage protection circuit, and does not work for asymmetric one. As another solution for protecting against rectifying effect of the lamp tube, by virtue of the asymmetry of positive and negative half-periods of the tube voltage, a direct current signal is extracted and supplied to an IC driving chip to stop an oscillator. In this technology, the voltage across both ends of the lamp tube is added after the positive and negative peaks are rectified. When the positive and negative peak voltage is asymmetric, a signal is sent to control the IC driving chip to stop oscillating and thereby implement protection. This solution will thus not work if the voltage is high and symmetric. Thus, it can be seen that the design of the lamp tube end-of-life protection circuit in the prior art are not all-sided, and the circuit is complicated. All of the circuits in the prior art cannot achieve protection substantially against the end of life of the fluorescent lamp.
A need exists for a simple and all-sided protection circuit which can shorten a fluorescent lamp in which rectifying effect and tube overvoltage occurs and has no influence on the other lamp tubes which are running normally. For abnormalities occurring at the end of life of the lamp tube, the present invention solves the problems in the prior art by detecting voltage waveform and amplitude of the lamp tube of the fluorescent lamp.
A fluorescent lamp end-of-life protection circuit in accordance with the present invention includes a lamp tube rectifying effect protection circuit and a lamp tube overvoltage protection circuit, characterized in that a circuitry of two series resistors and a capacitor in series is connected in parallel with the lamp tube, a cathode of a transient voltage suppresser is connected to one end of the lamp tube, its anode is connected to an anode of a diode, and a cathode the diode is connected to a common node of two series resistors; one end of a trigger diode is connected to a common node of the capacitor and the resistor, and the other end is connected to a gate terminal G of a triac; and a first electrode and a second electrode of the triac are connected to two ends of the lamp tube respectively.
The present invention can be sorted into a lamp tube rectifying effect protection circuit and a lamp tube overvoltage protection circuit according to its function. When rectifying effect occurs in the lamp tube, voltage across both ends of the lamp tube has asymmetric positive and negative half-wave waveforms in one period. The capacitor is charged by the tube voltage through two series resistors, therefore, a direct current component of the tube voltage is saved in the capacitor. The bi-directional trigger diode is turned on when the capacitor has saved a certain amount of energy. At the same time, the triac is turned on, and the lamp tube in which rectifying effect occurs can be protected thereby. When the lamp tube voltage increases rapidly and the positive and negative half-wave waveforms are still symmetric at the end of life of the lamp tube, the transient voltage suppresser is turned on. At this time, the tube voltage is applied to the resistor and the capacitor through the transient voltage suppresser and the diode. Therefore, there is enough energy in the capacitor to turn on the bi-directional trigger diode and the triac at the same time, and protection is obtained. In both cases as described above, when the bi-directional trigger diode is turned on, the triac is turned on. Because the first electrode and the second electrode of the triac are connected to two ends of the lamp tube respectively, the lamp tube is turned off when the triac is turned on, and a current flows to the triac so that neither current nor voltage is applied to the lamp tube which reaches its end of life. Accordingly, it is possible to prevent the cathodes at both ends of the lamp tube from overheating, and thus there does not exist hazard due to overheating of the lamp tube. With this technical solution, extra power consumption can be avoided and the electronic ballast can be protected.